No applicable.
The present invention relates to catalysts and processes for the catalytic partial oxidation of hydrocarbons (e.g., natural gas), for the preparation of a mixture of carbon monoxide and hydrogen using a supported metal catalyst. More particularly, the invention relates to catalysts and processes using catalysts comprising promoted nickel-based catalysts supported on magnesium oxide.
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population, and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or xe2x80x9csyngasxe2x80x9d). In a second step, the syngas is converted to hydrocarbons.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, proceeding according to Equation 1.
CH4+H2OCO+3H2xe2x80x83xe2x80x83(1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue.
The catalytic partial oxidation of hydrocarbons, e.g., natural gas or methane to syngas is also a process known in the art. While currently limited as an industrial process, partial oxidation has recently attracted much attention due to significant inherent advantages, such as the fact that significant heat is released during the process, in contrast to steam reforming processes.
In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2.
CH4+1/2O2CO+2H2xe2x80x83xe2x80x83(2)
This ratio is more useful than the H2:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol and to fuels. The partial oxidation is also exothermic, while the steam reforming reaction is strongly endothermic. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes. The syngas in turn may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch Synthesis.
The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. Difficulties have arisen in the past in making such a choice economical. Typically, catalyst compositions have included precious metals and/or rare earths. The large volumes of expensive catalysts needed by present day catalytic partial oxidation processes have placed these processes generally outside the limits of economic justification.
For successful operation at commercial scale, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, and the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Such high conversion and selectivity must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (xe2x80x9ccokexe2x80x9d) on the catalyst, which severely reduces catalyst performance. Accordingly, substantial effort has been devoted in this field to the development of catalysts allowing commercial performance without coke formation.
A number of process regimes have been proposed for the production of syngas via partial oxidation reactions. For example, the process described in U.S. Pat. No. 4,877,550 employs a syngas generation process using a fluidized reaction zone. Such a process however, requires downstream separation equipment to recover entrained supported-nickel catalyst particles.
A fixed bed reactor configuration would alleviate the catalyst degradation, but would require a pressure differential (pressure drop) to allow gas flow over the catalyst. To overcome the relatively high pressure drop associated with gas flow through a fixed bed of catalyst particles, which can prevent operation at the high gas space velocities required, various structures for supporting the active catalyst in the reaction zone have been proposed. U.S. Pat. No. 5,510,056 discloses a monolithic support such as a ceramic foam or fixed catalyst bed having a specified tortuosity and number of interstitial pores that is said to allow operation at high gas space velocity. The preferred catalysts for use in the process comprise ruthenium, rhodium, palladium, osmium, iridium, and platinum. Data are presented for a ceramic foam supported rhodium catalyst at a rhodium loading of from 0.5-5.0 wt %.
U.S. Pat. No. 5,648,582 also discloses a process for the catalytic partial oxidation of a feed gas mixture consisting essentially of methane. The methane-containing feed gas mixture and an oxygen-containing gas are passed over an alumina foam supported metal catalyst at space velocities of 120,000 hr.xe2x88x921 to 12,000,000 hr.xe2x88x921 The catalytic metals exemplified are rhodium and platinum, at a loading of about 10 wt %.
Catalysts containing Group VIII metals such as nickel on a variety of supports are known in the art. For example, V. R. Choudhary et al. (xe2x80x9cOxidative Conversion of Methane to Syngas over Nickel Supported on Low Surface Area Catalyst Porous Carriers Precoated with Alkaline and Rare Earth Oxides,xe2x80x9d J. Catal., Vol. 172, pages 281-293, 1997) disclose the partial oxidation of methane to syngas at contact times of 4.8 ms (at STP) over supported nickel catalysts at 700 and 800xc2x0 C. The catalysts were prepared by depositing NiOxe2x80x94MgO on different commercial low surface area porous catalyst carriers consisting of refractory compounds such as SiO2, Al2O3, SiC, ZrO2 and HfO2. The catalysts were also prepared by depositing NiO on the catalyst carriers with different alkaline and rare earth oxides such as MgO, CaO, SrO, BaO, Sm2O3 and Yb2O3.
U.S. Pat. No. 5,149,464 discloses a method for selectively converting methane to syngas at 650xc2x0 C. to 950xc2x0 C. by contacting the methane/oxygen mixture with a solid catalyst, which is either:
(a) a catalyst of the formula MxMxe2x80x2yOz where:
M is at least one element selected from Mg, B, Al, Ln, Ga, Si, Ti, Zr and Hf;
Ln is at least one member of lanthanum and the lanthanide series of elements,
Mxe2x80x2 is a d-block transition metal, and each of the ratios x/z and y/z and (x+y)/z is independently from 0.1 to 8; or
(b) an oxide of a d-block transition metal; or
(c) a d-block transition metal on a refractory support; or
(d) a catalyst formed by heating a) or b) under the conditions of the reaction or under nonoxidizing conditions.
The d-block transition metals are stated to be selected from those having atomic number 21 to 29, 40 to 47 and 72 to 79, the metals scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum and gold. Preferably Mxe2x80x2 is selected from Fe, Os, Co, Rh, Ir, Pd, Pt and particularly Ni and Ru. The exemplary conversions, selectivities, and gas hourly space velocities are relatively low however, while reaction temperatures are relatively high, and the effects of coke formation are not addressed.
U.S. Pat. No. 5,855,815 (Park) describes certain alkali-element promoted NiO catalysts supported on silicon-containing supports. These catalysts are used for CO2, reforming of methane with or without the addition of O2 and H2O.
E. Ruckenstein et al. (Applied Catalysis A: General 183:85-92 (1999); Ind Eng. Chem. Res. 37:17441747 (1998)) describe certain solid solution catalysts for partial oxidation of methane and CO2 reforming of methane containing NiO supported on MgO, SiO2, AlO3 or La2O3.
U.S. Pat. No. 5,744,4:19 (Choudhary et al.) describes certain supported Ni and Co catalysts, with noble metal promoters, that are employed for the production of syngas by partial oxidation with oxygen or oxidative steam and/or CO2 reforming with oxygen of methane or light hydrocarbons.
U.S. Pat. No. 5,368,835 (Choudhary et al.) and U.S. Pat. No. 5,338,488 (Choudhary et al) describe certain Ni-based composite catalysts containing various rare earth or alkaline earth elements, for catalyzing the production of synthesis gas by oxidative conversion of methane.
V. R. Choudhary et al. (J. Catalysis 178:576-585 (1998)) describe processes for the oxidative conversion of methane to syngas catalyzed by NiO supported on various oxides such as CaO, MgO and rare earth oxides. Support effects on NiO in the partial oxidation of methane to syngas are discussed. The beneficial effects of adding Co to certain NiO catalysts for oxidative conversion of methane to syngas have also been described (Chaudhary et al. Applied Catalysis A: General 162:235-238 (1997)).
There have been previous attempts at synthesis gas production by catalytic partial oxidation to overcome some of the disadvantages and costs of steam reforming. In EPO 303438, for example, the asserted advantages of the process disclosed therein are described as being relatively independent of catalyst composition., i.e., xe2x80x9c . . . partial oxidation reactions will be mass transfer controlled. Consequently, the reaction rate is relatively independent of catalyst activity, but dependent on surface area-to-volume ratio of the catalyst.xe2x80x9d No promoters are suggested. In that process, a monolith catalyst is used with or without metal addition to the surface of the monolith at space velocities of 20,000-500,000 hrxe2x88x921. The suggested metal coatings of the monolith are palladium, platinum, rhodium, iridium, osmium, ruthenium, nickel, chromium, cobalt, cerium, lanthanum, and mixtures thereof, in addition to metals of the groups IA, IIA, III, IV, VB, VIB, or VIIB. Steam is required in the feed mixture to suppress coke formation on the catalyst, and significant quantities of carbon dioxide, steam, and C2+hydrocarbons are produced in addition to the desired CO and H2.
None of the existing processes or catalysts provide a partial oxidation catalyst or process capable of high conversion and high selectivity capable of operation with very low coke formation. Accordingly, there exists a need for a process and catalyst for the catalytic partial oxidation of hydrocarbons, particularly methane, that has low coke formation, high conversions of methane and high selectivities to CO and H2, and that is economically feasible at commercial-scale conditions.
The present invention provides a process and catalysts for the catalytic partial oxidation of a hydrocarbon feedstock, and a method for preparing the catalysts. The new catalysts and processes overcome many of the deficiencies of conventional partial oxidation catalysts and processes for producing synthesis gas. The syngas production process generally comprises the catalytic partial oxidation of a hydrocarbon feedstock by contacting a feed stream comprising a hydrocarbon feedstock and an oxygen-containing gas with a catalyst in a reaction zone maintained at conversion-promoting conditions effective to produce an effluent stream comprising carbon monoxide and hydrogen.
In accordance with the :present invention, certain embodiments of the catalyst employed in the process comprises about 1 wt. % to about 50 wt. % nickel supported on magnesium oxide and about 0.1 wt. % to about 10 wt. % of at least one promoter selected from the group consisting of manganese, molybdenum, tungsten, tin, rhenium, bismuth, indium and phosphorus.
In some embodiments, the catalyst comprises nickel, magnesium oxide, and a promoter selected from the group consisting of manganese, molybdenum, tungsten, tin, rhenium, bismuth, indium, phosphorus (metals and metal oxides), and combinations thereof.
In some embodiments, a supported partial oxidation catalyst comprising nickel, magnesium oxide, and a promoter selected from the group consisting of manganese, molybdenum, tungsten, tin, rhenium, bismuth, indium, phosphorus (metals and metal oxides), and combinations thereof, is provided.
Also in accordance with the invention, a method is provided for preparing a partial oxidation catalyst that operates at relatively low temperatures, and has high activity and selectivity for producing CO and H2 from methane. In some embodiments the method comprises preparing an aqueous solution of a nickel salt and a promoter, impregnating a magnesium oxide solid with the solution, calcining the impregnated solid, and reducing the calcined solid.
Also in accordance with the present invention is provided a method of converting a reactant gas mixture comprising a C1-C5 hydrocarbon and oxygen into a product gas mixture comprising CO and H2 by a net partial oxidation process. In some embodiments the method includes contacting the reactant gas mixture at partial oxidation promoting conditions of temperature and pressure with a supported catalyst comprising nickel, magnesium oxide, and one or more elements selected from the group consisting of manganese, molybdenum, tungsten, tin, rhenium, bismuth, indium and phosphorus.
Still other embodiments, features and advantages of the present invention will appear from the following description.
The term xe2x80x9ccatalytic partial oxidationxe2x80x9d when used in the context of the present syngas production methods, in addition to its usual meaning, can also refer to a net catalytic partial oxidation process, in which hydrocarbons (comprising mainly methane) and oxygen are supplied as reactants and the resulting product stream is predominantly the partial oxidation products CO and H2, rather than the complete oxidation products CO2 and H2O. For example, the preferred catalysts serve in the short contact time process of the invention, which is described in more detail below, to yield a product gas mixture containing H2 and CO in a molar ratio of approximately 2:1. Although the primary reaction mechanism of the process is partial oxidation, other oxidation reactions may also occur in the reactor to a lesser or minor extent. As shown in Equation (2), the partial oxidation of methane yields H2 and CO in a molar ratio of 2:1.
A process according to the present invention may be used to prepare a mixture of carbon monoxide and hydrogen from any gaseous hydrocarbon having a low boiling point, such as methane, natural gas, associated gas, or other sources of light hydrocarbons having from 1 to 5 carbon atoms. The new process is characterized by low coke formation, high conversions of methane and high selectivities to CO and H2 products, and is economically feasible at commercial-scale conditions.
The hydrocarbon feedstock is in the gaseous phase when contacting the catalyst. Natural gas is mostly methane, but it can also contain up to about 25 mole % ethane, propane, butane and higher hydrocarbons. The new process may be used for the conversion of gas from naturally occurring reserves of methane, which can also contain carbon dioxide, nitrogen, hydrogen sulfide, and other minor components. Preferably, the feed comprises at least 50% by volume methane, more preferably at least 75% by volume methane and, most preferably at least 80% by volume methane.
The hydrocarbon feedstock is contacted with the catalyst as a mixture with an oxygen-containing gas, preferably pure oxygen. The oxygen-containing gas may also comprise steam and/or CO2 in addition to oxygen. Alternatively, the hydrocarbon feedstock is contacted with the catalyst as a mixture with a gas comprising steam and/or CO2.
A methane-containing feedstock and the oxygen-containing gas are preferably mixed in such amounts to give a carbon (i.e., carbon in methane) to oxygen (i.e., oxygen) ratio from about 1.25:1 to about 3.3:1, more preferably from about 1:3:1 to about 2.3:1, and most preferably from about 1.5:1 to about 2.2:1, especially the stoichiometric ratio of about 2:1.
The process of the present invention may be operated at atmospheric or super-atmospheric pressures, with the latter being preferred. The process may be operated at pressures of from about 100 kPa to about 12,500 kPa, and preferably from about 130 kPa to about 10,000 kPa.
The process of the present invention may be operated at temperatures of about 600xc2x0 C. to about 1300xc2x0 C., and preferably about 700xc2x0 C. to about 1100xc2x0 C. The hydrocarbon feedstock and the oxygen-containing gas may be pre-heated before contact with the catalyst, preferably the reactant gas mixture is pre-heated to a temperature of about 300-700xc2x0 C., more preferably about 525xc2x0 C.
The hydrocarbon feedstock and the oxygen-containing gas can be passed over the catalyst at a variety of space velocities. Typical space velocities for the process, stated as normal liters of gas per kilogram of catalyst per hour, are from about 20,000 to about 100,000,000 NL/kg/h, preferably from about 50,000 to about 50,000,000 NL/kg/h. Ceramic foam monoliths are typically placed before and after the catalyst as radiation shields. The inlet radiation shield also typically aids in uniform distribution of the feed gases.
The catalyst used in the process of the present invention preferably comprises about 1 wt. % to about 50 wt. % nickel supported on magnesium oxide, and about 0.1 wt. % to about 10 wt. % of at least one promoter selected from the group consisting of manganese, molybdenum, tungsten, tin, rhenium, bismuth, indium and phosphorus (as metals or metal oxides), and mixtures thereof. If desired, in the presence of a preferred promoters, additional promoters may be included such as metal or metal oxides of cobalt, rare earth elements, chromium, iron, vanadium, copper, alkali or alkaline earth metals (i.e., Group IA, IIA), and combinations thereof. Preferably, the catalyst is prepared using any of the techniques known to those skilled in the art, such as: impregnation, sol-gel methods, and co-precipitation. In an impregnation method of preparation, magnesium oxide is preferably contacted with solutions of a nickel salt and one or more promoter salts. The nickel salt and one or more promotor salts may be contained in the same solution and loaded onto the support in a single step, or they may be applied to the support as separate solutions, drying the support after each impregnation step. The loaded or impregnated magnesium oxide is then dried and calcined.
The catalyst composition may be supported on a carrier selected from the group consisting of spinels, perovskites, magnesium oxide, pyrochlores, brownmillerites, zirconium phosphate, magnesium stabilized zirconia, zirconia stabilized alumina, silicon carbide, yttrium stabilized zirconia, calcium stabilized zirconia, yttrium aluminum garnet; alumina, cordierite, ZrO2, MgAl2O4, SiO2 or TiO2, preferably MgO.
The support or carriers may be in the form of powders, mesh sized particles, reticulated foams, honeycombs, perforated plates, corrugated supports, or any other support that may be desired according to the preference of those skilled in the art. Preferred supports have a tortuosity of about 1.0.
The catalyst composition, with or without a support material, may be applied to a support by any of the other methods well known in the art, such as impregnation, wash coating, adsorption, ion exchange, precipitation, co-precipitation, deposition precipitation, sol-gel method, slurry dip-coating and microwave heating. Alternatively, the catalyst components may be extruded, with or without a ceramic support composition, to prepare a three-dimensional form such as a honeycomb or a foam. Suitable foams for use in the preparation of the catalyst preferably have from 30 to 150 pores per inch (12 to 60 pores per centimeter).
The supports for use in the present invention are preferably in the form of monolithic supports or other configurations having longitudinal channels or passageways permitting high space velocities with a minimal pressure drop. Such configurations are known in the art and described in, for example, Structured Catalysts and Reactors, A. Cybulski and J. A. Moulijn (Eds.), Marcel Dekker, Inc., 1998, p. 599-615 (Ch. 21, X. Xu and J. A. Moulijn, xe2x80x9cTransformation of a Structured Carrier into Structured Catalystxe2x80x9d) hereby incorporated herein by reference in its entirety.
Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The following embodiments are to be construed as illustrative, and not as limiting the disclosure in any way whatsoever.